This invention relates to an etching gas suitable for selectively etching silicon nitride and a process for selectively etching silicon nitride with the etching gas, and more particularly to an etching gas capable of selectively etching silicon nitride with a higher selectivity or at a higher etching rate than silicon oxide and silicon and a process for etching with the etching gas.
As is well known, dry etching of silicon or its compound is carried out with a reacting gas, for example, CF.sub.4, CF.sub.4 +O.sub.2, NF.sub.3, SF.sub.6, CHF.sub.3, CF.sub.4 +H.sub.2, etc. However, in the conventional dry etching with these reacting gases, etching rates of Si, SiO.sub.2 and Si.sub.3 N.sub.4 with a reacting gas such as CF.sub.4, CF.sub.4 +O.sub.2, NF.sub.3 or SF.sub.6 are such that the etching rate of Si is highest, and the etching rate is decreased in the order of Si.sub.3 N.sub.4 and SiO.sub.2. With CHF.sub.3 or CF.sub.4 +H.sub.2 as the reacting gas, the etching rates of SiO.sub.2 and Si.sub.3 N.sub.4 are increased, as compared with that of Si, but the etching rate ratio of Si.sub.3 N.sub.4 to SiO.sub.2 is about 2-3, and thus Si.sub.3 N.sub.4 cannot be selectively etched.
Thus, a reacting gas such as CF.sub.4 +O.sub.2 or SF.sub.6 has been used for selective etching of Si.sub.3 N.sub.4, but the etching rate of Si is so high that the substrate Si is liable to be etched and thus there is a risk of considerable damages. To prevent such a risk, a SiO.sub.2 film must be provided between the Si.sub.3 N.sub.4 film and the substrate Si, and furthermore the SiO.sub.2 film must be thicker owing to a low selectivity between SiO.sub.2 and Si.sub.3 N.sub.4. That is, it has been so far difficult to selectively dry-etch a Si.sub.3 N.sub.4 film with a higher selectivity or at a higher etching rate than Si and SiO.sub.2, and this has been a serious trouble in forming semiconductor devices.